Polyethylenes with a multimodal molecular weight distribution (“MWD”) are of interest because they can combine the processability attributes of low molecular weight polymers with the mechanical properties of high molecular weight polymers. It is thought that adding an ultra-high molecular weight polyethylene (“UHMWPE”) component to polyethylene resins of lower molecular weight may improve the properties of that resin and retain the processing properties normally associated with lower molecular weight polyethylene. However, simple blending of a UHMWPE and a linear low density polyethylene may result in both melt and solid phase separation (See Chen, Y.; Zou, H.; Liang, M.; Liu, P. J. App. Polym. Sci. 2013, 129, 945-953.)
The ability to produce a UHMWPE in the same reactor as a lower molecular weight material could provide a cost-effective way of producing a multimodal resin with the different molecular weight components already intimately mixed. To enable this, it would be advantageous if a catalyst capable of producing UHMWPE could be cosupported with a metallocene catalyst such that the two catalysts did not interfere with one another. However, cosupportation of catalysts is complex and it is difficult to select catalyst combinations that are both compatible (i.e., do not interfere with each others' ability to produce polymer) and produce the desired product.
Even when cosupportation is successful, it typically leads to polymers with a MWD having a number of modes equal to the number of catalysts supported. Thus, to produce a polymer with a trimodal MWD, one would theoretically need to find three catalysts that are compatible and can be successfully activated on the same support. The ability to reduce the number of catalysts on the support and still maintain the n-modal capability of the cosupported catalyst system (n>2) would simplify the catalyst system and preparation processes and decrease the possibility of the catalysts interfering with each other.
Half sandwich chromocenes are disclosed in DE 19710615, WO 2012/040147, US 2013/0225820, US 2010/0267901, and CN 102070732. In particular, J. Organometallics 2000, 19, 388-402 (Dohring, et al.) discloses ethylene(cyclopentadienyl) (pyrrolidine)chromium dichloride.
Further, WO 2006/052232 discloses a catalyst comprising a chromocene and bis(n-butyl cyclopentadienyl)zirconium dichloride to produce broad Mw/Mn, high molecular weight polyethylene.
WO 2011/089017 discloses the preparation of high molecular weight polyethylene using a catalyst of a half sandwich indenyl chromocene and a hafnocene.
WO 2008/140875 discloses a combination of metallocene with chromium catalyst on the same support.
Other references of interest include: US 2012/0059134, WO 2012/067777, WO 2014/099307, WO 2015/191290, and Macromol. Rapid Comm., 2010, 31, 1359-1363 (Kurek, et al.).
There is still a need for new and improved catalyst systems for the polymerization of olefins to achieve specific polymer properties, such as multi-modality, without comprising polymer processability. There is also a need for polymer catalyst systems having n-modal capability (n>2) that are simple and cost effective to prepare and use in polymerization processes.